This invention relates to preparing lithiated vanadium oxide coatings of optical quality useful, e.g., as ion-storage counterelectrodes for electrochromic devices.
Electrochromic materials undergo a color change upon oxidation or reduction. In an ion-intercalation electrochromic device, an electrochromic material and an ion-storage counterelectrode material are separated by an ion-conducting electrolyte. The optical properties of the electrochromic material change when ions (e.g., hydrogen ions or metal ions such as lithium ions) intercalated within the structure of the ion-storage material are removed and intercalated within the structure of the electrochromic material in response to an applied electric potential. The ions are removed and returned to the ion-storage material by reversing the polarity of the applied potential, thereby returning the electrochromic material to its original optical state.
A preferred ion-storage material is lithiated vanadium oxide. These materials have been prepared in a number of different ways. According to one process, a solution phase precursor is coated onto an electronically conductive substrate (e.g., an FTO-coated glass substrate) and then heated to generate a vanadium oxide layer. This layer is then charged in a separate step by negatively biasing the vanadium oxide-coated substrate while it is submerged in a lithium ion-containing electrolyte solution to electrochemically insert lithium ions into the oxide layer.
Other processes for preparing lithiated vanadium oxide include physical vapor deposition processes such as sputtering, laser ablation, pulsed laser deposition, and evaporation. These processes, which are typically performed in a vacuum chamber, can be adjusted to yield lithiated material directly, in which case it is not necessary to insert lithium ions in a separate charging step. However, unlike the solution phase processes, many physical vapor deposition processes are not well-suited to producing uniform large-area oxide coatings. They are also relatively expensive.